The present invention relates to an image reading apparatus and method, more particularly to an image reading apparatus and method used for a digital duplicator, a facsimile and so on. In the image reading apparatus, generally speaking, an image drawn on a sheet of paper is read by a scanner, then the read image is reproduced on another sheet of paper. The image is, for example, letters, pictures, and other drawings drawn on a sheet of paper. The image reading apparatus and method has an improved method for correcting a dark-state output level of an array of photoelectric elements provided therein.
A metal halide lamp, a fluorescent lamp, an array of light emitting diodes, and other means, have been used for a light source of a scanner provided in a image reading apparatus. It is necessary in an image reading apparatus to correct errors included in an output signal. The errors are due to the following causes. One of the causes is variation of an illuminance of a light source such as those light sources as mentioned above. Such variation is caused by variation in characteristics of the light source per each product. The variation is caused by change of temperature thereof, due to the aging thereof, and other causes. Another of the causes is variation in attenuation properties of optical devices (attenuation is caused by transmission or reflection of light thereof) per each product. And further cause is variation of sensitivities of photoelectric-conversion-elements per each product.
The analogue-digital conversion is performed on an output signal, which signal is provided from photoelectric-conversion-elements. The analogue-digital conversion is needed to be performed with a large dynamic range so as to perform accurate conversion by the methods of this invention. The above mentioned errors in signal due to the above mentioned causes are needed to be corrected most effectively. From the point of view of the above, methods for obtaining an image signal by means of analogue-digital conversion are roughly categorized into the following 2 methods.
(1) A first method of the 2 methods is that: an illuminance of a light source is controlled so as to set an output level of photoelectric conversion elements to a predetermined level. The output level is obtained at a time when a reference-white-board is read. PA1 (2) A second method of the 2 methods is that: a gain-controlled amplifier is provided for an image processing apparatus so as to set an output level of photoelectric conversion elements to a predetermined level. The output level is obtained at a time when a reference-white-board is read. PA1 a light source for applying light onto an original image; PA1 photoelectric conversion means or step for converting light reflected from the original image into an electric signal and for providing the electric signal; PA1 level-controlling-quantity setting means or step for setting a level-controlling-quantity so as to make the level-controlling-quantity correspond to an electric signal provided from the photoelectric conversion means; PA1 level controlling means or step for controlling an electric signal provided from the photoelectric conversion means so as to make a level of the electric signal correspond to the level-controlling-quantity; PA1 dark-state-output correction means or step for correcting output of the photoelectric conversion means by means of a dark-state-output; and PA1 controlling means or step for obtaining the dark-state-output by a process such that an output of the photoelectric conversion means is obtained during a dark-state, during the dark-state, wherein there is almost no light being applied to the photoelectric conversion means, the output of the photoelectric conversion means is controlled by means of the level controlling means with a level-controlling-quantity, the level-controlling-quantity being the same as a level-controlling-quantity used during a previous-image-reading-state. PA1 image-reading-condition detecting means for detecting an image-reading-condition of said image reading apparatus; PA1 dark-state-output correction means for correcting output of the photoelectric conversion means by means of the dark-state-output; and PA1 controlling means for forecasting a level-controlling-quantity used for obtaining a dark-state-output, forecasting being performed by means of converting an output of the image-reading-condition detecting means into a corresponding level-controlling-quantity by means of a predetermined function; the controlling means also obtaining the dark-state-output by a process such that an output of the photoelectric conversion means is obtained during a dark-state, wherein the dark-state almost no light is applied to the photoelectric conversion means, the output of the photoelectric conversion means is controlled by means of the level controlling means with a level-controlling-quantity, the level-controlling-quantity being obtained by forecasting.
In the first method of the above 2 methods, the method of controlling an illuminance, not only does a means for controlling an illuminance of a light source incur a high cost, but also an image-reading-time becomes long. A reason of this long image-reading-time is described below. That is, a process for the controlling of an illuminance takes a lot of time. Therefore, generally speaking, the second method is used. The second method is the method of using a gain-controlled amplifier and other apparatus as needed for correcting the errors by means of signal processing with a predetermined apparatus.
In an image reading apparatus, photoelectric conversion elements have a certain output level even in a dark-state. The dark-state means that no light is applied to the array of photoelectric elements. This certain output level may degrade a stability of an image signal output. Or the output level may degrade a tone-level-resolving ability of the scanner or the image reading apparatus. The image signal output thus needs to be corrected by various methods to prevent the interference. The interference may be caused by the above mentioned certain output level of the photoelectric conversion elements. This correction will be called the `dark-state-output correction` hereinafter. For example, an output image signal, having a high accuracy, can be obtained in a image reading apparatus by the following construction. That is, the image reading apparatus will have a high tone-level-resolving ability if correction of errors, which errors is included in the output image signal, is performed. This correction is performed at a time when an original image drawn on a sheet of paper is read by the scanner. The dark-state-output correction is performed in the following manner. All output data of an array of photoelectric-conversion-elements are used. Those output data are all pixel-outputs along an array direction (for example, outputs of 5000 pixel elements, that is, 5000 pieces of pixel-outputs). The all pixel-output is obtained at a time when a reference-black-board is read by the scanner. This reading method is used instead of using such method as reading in the dark-state. Then, the all pixel-output are stored in a memory after the data are converted into digital data.
However, the array of photoelectric-conversion-elements cannot provide predetermined output levels respectively corresponding to a white color and a black color under the following condition. The condition is that the reference-white-board and the reference-black-board are successively read by the scanner. A reason, why the predetermined output levels cannot be obtained, is described below. The array of photoelectric elements suffers influences of the other tones located adjacent to the tone to be read. This condition is formed in an arrangement where the reference-white-board and the reference-black-board are located to be adjacent to each other. To prevent the influence of the other tones located to be adjacent to the tone to be read, the following arrangement is needed. That is, a sufficient large distance between both the boards along a direction is to be made. This distance is made to be along a line where the scanner runs when it is reading an original image drawn on a sheet of paper to be read. However this arrangement of the boards causes enlargement of the outer dimensions of the image reading apparatus. This arrangement also causes an increase of time taken for reading of the original image. This time-elongation is caused by an increase in the time taken for a provisional running of the scanner. The provisional running of the scanner is performed to read the both reference boards. This provisional running is performed before the scanner reads the original image. The boards are located so as to be apart from each other a sufficient distance as mentioned above.
To overcome problems such as mentioned above, conventional technology has been disclosed in Japanese Laid-Open Patent No. 62-73869 and Japanese Laid-Open Patent No. 62-235871. In the technology, a reference-black-output-level(output level during the dark state) is obtained. The reference-black-output-level corresponds to the predetermined output level obtained by reading the reference-black-board.
FIG.1 shows a diagrammatic construction of an optical system including a scanner, which scanner is provided in an image reading apparatus. The image reading apparatus accords to technology relating to the present invention.
In the construction shown in FIG.1, light is applied by means of a light source 103 onto an original image drawn on a sheet of paper placed on a contacting glass plate 101. Light reflected on the original image on the sheet of paper is then reflected by a first mirror 104, and then by second and third mirror 105a and 105b respectively. Light is then applied to a lens 106. Light then focuses a CCD (Charge Coupled Device Image Sensor) 107. A scanner comprises the light source 103, the mirrors 104, 105a, and 105b. The scanner reads a reference-white-board 102 before reading the original image on the sheet of paper. This reading of a reference-white-board 102 is performed for the purpose of a shading-correction-process. This shading-correction-process is performed for performing a correction on image-output-signal of the CCD 107 so as to prevent undesired shading being drawn on a duplicated image (if the image reading apparatus is being used for a duplicator). This undesired shading drawn is caused by variation in an illuminance of the light source 103 and other causes.
The CCD 107 comprises a linear array of photoelectric-conversion-elements whose location is indicated in FIG.1 and is oriented perpendicular to the page of FIG.1. The scanner runs along a transverse direction, as in FIG.1. The running of the scanner is performed between a position of the light source 103, the mirrors 104,105a, and 105b drawn by solid line and another position of the light source 103, the mirrors 104, 105a, and 105b drawn by broken line.
FIG.2 shows a simplified block diagram of an image-signal-processing system provided in the above mentioned image reading apparatus. In FIG.2, an output signal provided from the CCD 107 is applied to a sample and hold apparatus (S/H) 203. The output signal provided from the CCD 107 includes reset noise caused by an extracting-electrons-from-CCD process. The reset noise is removed by means of the S/H 203 by sampling an appropriate part of the output signal. A gain-controlled-amplifier (GC AMP) 204 has a construction shown in FIG.3. A gain of the GC AMP 204 can be controlled so as to be `1` through `15` by means of 4 pieces of control signal P.sub.1 through P.sub.4. In an analogue-digital converter (ADC) 205, an output signal of the GC AMP 204 is converted into digital data having 6-bits. In a subtracter 206, the dark-state-output-correction is performed by subtracting a dark-state-output from an image-reading-output.
The dark-state-output is stored in a memory 207 at a time when the light source 103 is turned off. The dark-state-output comprises all outputs of all photoelectric-conversion-elements of the CCD 107. All these outputs are all pixel-outputs of pixels corresponding to the array of the photoelectric-conversion-elements, which array is located along a line whose direction is perpendicular to the page on which FIG.1 is drawn. The all pixel-output is respectively written into respective addresses of the memory 207. On the other hand, when the light source 103 is turned on, the dark-state-output stored in the memory 207 is read from respective addresses of the memory 207. Each address of the memory 207 corresponds to respective one of the addresses of pixel-output of the CCD 107. The pixel-output corresponds to an original image on a sheet of paper, which original image is illuminated by means of the light source 103.
A peak-detecting-apparatus 208 detects a maximum value from values of the respective outputs of elements of the array of the photoelectric-conversion-elements, which array comprises the CCD 107. The peak-detecting-apparatus 208, in a normal state, provides the values (1, 1, 1, 0) as P.sub.1 through P.sub.4 respectively to the GC AMP 204. A gain of the GC AMP 204 is set by means of these values so as to be a reference gain.
FIG.4 shows a table for a use of obtaining a gain of the apparatus shown in FIG.3. The gain value is set by control signals P.sub.1 through P.sub.4, which respectively control open-close-operations of switches SW.sub.1 through SW.sub.4. This control is performed so that: the switch is opened if value `0` of the control signal is applied thereto; and the switch is closed if value `1` of the control signal is applied thereto. In the table of FIG.4, values `1` or `0`, indicated in the 4 lefthand columns of the table, respectively show values of the signal P.sub.1 through P.sub.4. And values, indicated in the righthand large column of the table, show the gain values. Some of the gain values are expressed as fractions, for example, `15/14` and so on. These gain values are obtained if the switches SW.sub.1 through SW.sub.4 are controlled as per the control signals P.sub.1 through P.sub.4 as mentioned above. The values of the control signals P.sub.1 through P.sub.4 are respective values indicated in the 4 lefthand columns of the table on each line thereof. The gain value is a value indicated in the righthand column of the table in the same line as the above mentioned line.
In the normal state, values of the control signals are (1, 1, 1, 0) as mentioned above. Then, the gain value of the apparatus shown in FIG.3 (the GC AMP 204) becomes `15/14` as per a second line from a top line of the table of FIG.4.
The apparatus 208 provides control signals to the GC AMP 204 soon after the light source 103 is turned on. The control signals have 4-bits. These bits are the most significant 4-bits of a peak-value (having 6-bits provided by means of the ADC 205, which has 6-bit output as mentioned above). This peak value is detected as a result of the scanner reading the reference-white-board 103. The apparatus 208 holds the control signals, which are being applied to the GC AMP 204, until one image-reading-operation is finished. Normally, this one image-reading-operation means that the scanner scans one sheet of paper, which paper had been drawn an original image.
For example, if the peak value is obtained in the apparatus 208, assumed as a value `42`, in the decimal system(`1, 0, 1, 0, 1, 0` in the binary system). Then values (1, 0, 1, 0) P.sub.1 through P.sub.4, as control signals, are provided from the apparatus 208. These (1, 0, 1, 0) are the first 4-bits of the above mentioned (1, 0, 1, 0, 1, 0). Then a gain value of the GC AMP 204 becomes `15/10` as per a sixth line from the top line of the table of FIG.4. The gain value of the GC AMP 204 has been `15/14` as mentioned above, before a process such that the apparatus 208 provides a control signals (1, 0, 1, 0) to the GC AMP 204. The above mentioned peak value `42` has been obtained from an output signal. This signal is obtained as a result of processes, including a process performed by means of the GC AMP 204, which AMP 204 has the gain `15/14`.
After the process, the gain value of the GC AMP 204 is altered to `15/10`. Thus, a peak value now obtained is obtained from an output data, which is obtained as a result of processes, which include a process performed by means of the GC AMP 204. Now the AMP 204 has a gain value of `15/10`. Therefore, the peak value now obtained is obtained by the following expression: EQU {42.div.(15/14)}.times.(15/10)=58.8 (1).
This value `58.8` is very near to the value `64`. On the other hand, a value `64` is a maximum value that is able to be expressed by 6 bits in the binary system (2.sup.6 =64). This value `64` is also a maximum value, which value is able to be treated by the ADC 205. The ADC 205 has a 6-bit output as mentioned above. That is, the nearer that a peak value detected by the apparatus 208 becomes to the ADC 205 maximum, the larger that dynamic range usable in the ADC 205 becomes. The ADC 205 can perform an analogue-digital conversion-process in this dynamic range. As a result, the image reading apparatus uses this large dynamic range of the ADC 205 more effectively while scanning an original image on a sheet of paper and then forming a more accurate image-output-signal.
A controller 212 controls respective operation timing of a scanner driving motor(not shown in the drawings) for driving the scanner so as to drive it in the forward or reverse directions or stop it as shown in FIG.5A. Also the controller 212 controls timing of turning on and off of the light source 103 as shown in FIG.5B. The controller 212 also controls timing of sampling and holding of the S/H 203 as shown in FIG.5C. The controller 212 also controls timing of data storing in the memory 207. Also the controller controls a timing of making the peak detecting apparatus 208 deliver the controlling signal. The controller 212 also controls the peak detecting apparatus 208 so as to make it deliver the controlling signal (1, 1, 1, 0) to the GC AMP 204. This controlling signal results in setting the reference gain Go 15/14) thereto as shown in FIG.5D.
FIGS.5A through 5D show operation time charts of the above mentioned first embodiment of an image reading apparatus. In FIG.5D, Gn-1 indicates a gain, at which gain an output-image-signal has been processed by the GC AMP 204 during a last-time-reading-image-state. Go indicates the above mentioned reference gain `15/14` as a result of the control signal (P.sub.1, P.sub.2, P.sub.3, P.sub.4 = 1, 1, 1, 0). And Go indicates a gain, in which gain an output-image-signal has been processed by the GC AMP 204 during a present-time-reading-image-state.
The problem of the image reading apparatus is described below. If a peak value, which was obtained from an output-image-signal for a pixel, was `2` in the decimal system, the value `2` was then stored in the memory 207. The output-image-signal is provided by a photoelectric-conversion-element of the CCD 107. The output-image-signal results from processes, which processes includes a process by means of the GC AMP 204. The AMP 204 has the reference gain Go (`15/14`) before turning on the light source 103. On the other hand, a gain Gn of the GC AMP 204, during the present-time-image-reading-state, is `15/10`. The gain Gn of GC AMP 204 during the present-time-image-reading-state is not the same as the gain Go during the dark-state. An output-image-signal as a pixel-output, which output is provided from the element of the CCD 107, is processed by the GC AMP 204 in gain Gn during the present-image-reading-state. The gain Gn is different from the gain Gn. The output-image-signal as a pixel-output, which is provided from the element of the CCD 107, is processed by the GC AMP 204 in the gain Go during the dark-state. The output-image-signal, which has been obtained during the dark-state, is used as the above mentioned reference-black-output-level. This reference-black-output-black-level is used for correction of an image-output-signal so as to obtain high stability of an image-output signal and high-tone-level-resolving-ability. The reference-black-output-level is used instead of using a reference-black-board as mentioned above. An image-output-signal processed by means of the GC AMP 204. However, conditions are different between during the present-image-reading-state and during the dark-state due to the difference of gains of the AMP 204. That is, the bases of obtaining an image-output-signal are different for an image-output signal to be corrected and for the above mentioned reference-black-level. The image-output-signal will be corrected by means of the reference-black-output-level. In the condition, it is not possible to obtain the high stability of either image-output-signal or the high-tone-level-resolving-ability. A reason for this is that an accurate correction is performed on a condition that both bases of obtaining output are the same. One of the bases is the basis of obtaining the reference-black-output-level obtained during the dark-state, the reference-black-output-level is used for correction(dark-state-output-correction). Another of the bases is the basis of obtaining an image-output-signal obtained during the image-reading-state to perform the dark-state-output-correction.
To overcome the above mentioned problem, it is necessary to know Go before the present-image-reading-state is started. This gain is used for processing an image-output-signal during the present-image-reading-state. Then, this gain is set on the GC AMP 204 for processing an image-output signal during the dark-state so as to use it to obtain the reference-black-output. Using this gain for obtaining the black-reference-output-level makes it possible to equalize the bases of obtaining the image-output during the dark-state and during the image-reading-state with each other. The situation, where the above mentioned equalizing is not realized, results in a degraded dark-state-output-correction.
However, as mentioned above, the gain Gn cannot be obtained unless the light source 103 is turned on, then, in this example, the peak value `42`, in the decimal system, is obtained, then the gain Gn, in this example, Gn being `15/10`, is obtained as mentioned above. Also, this turning on of the light source 103 is needed to be performed just before the operation of reading the original image on the sheet of paper in the image reading apparatus. If a certain time elapses before the operation of reading the original image on the sheet of paper, a peak value may change, the peak value being applied by the apparatus 208 to the GC AMP 204 just before the operation of reading the original image, this peak value being, in this example, `42` as mentioned above. This change of the peak value may be due to a change of a condition of the image reading apparatus, for example, change of an illuminance of the light source 103 caused by change of temperature thereof. This change of the peak value, due to time elapsed before the operation of reading the original image, causes the same problem as in the related process of not turning on the light source 103 before the image-reading-state, due to change of gain of the GC AMP 204. The change of gain of the GC AMP 204 means the change of the basis of obtaining the image-output-signal between during the dark-state and during the image-reading-state.
Therefore it is necessary to perform an action such as turning on the light source 103 before the image-reading-state just before the image-reading-state. That is, this action is to be performed after an operator performs an operation on the image reading apparatus so as to make the image reading apparatus start the operation of reading the original image. If the light source 103 is turned on, after this operation being performed by the operator, and also before the image-reading-state, a certain time loss is thus incurred between the time of the operation being performed by the operator and the times of reading the original image in the image reading apparatus. This time loss incurred before reading the original image in an image reading apparatus prevents the satisfaction of a current request of users thereof, the current request being to provide duplicators, facsimiles, etc. having a higher processing speed.